This invention relates to connectors for integrated circuit chips and multi-chip substrates and, more particularly, to such a connector which provides for high density contacts in a separable assembly.
Integrated circuits (IC's) are conventionally carried on silicon modules which are like miniature printed circuit boards on which an interconnection pattern (in one or more layers) and contact pads are deposited. Such substrates may also incorporate active devices, line drivers, receivers, etc. A module may contain several IC chips which are mounted on the substrate by means of flip chip, wire bond or tape automated bonding methods, for example. This technology results in a very high interconnection density on the module. For example, the flip chip technology utilizes only 0.001" high solder bumps on centers as small as 0.004" to yield an extremely dense package. The modules are then interconnected via a printed circuit board. For reasons of minimizing the impact of signal propagation delay, such modules must be interconnected in as small an area as possible. Accordingly, it is an object of this invention to provide an assembly for the high density connection of an IC chip module to a printed circuit board or similar substrate.
In terms of connector density, the technology of connector assemblies has progressed from plated-through hole mounted contacts, to pin and socket combinations, to surface soldering, to the current use of interposers, the latter being exemplified by U.S. Pat. No. 4,699,593, issued on Oct. 13, 1987 and assigned to the assignee of the present invention. The interposer and similar methods typically use an insulator as a means of positioning or orienting contacts, which typically are spring members. However, when the dimensions of the plastic material separating adjacent contacts is below certain limits, the traditional technology of molding plastic becomes inadequate. There is a limitation of the space between core pins in the mold into which the plastic will flow, provide an adequate packing density and retain at least some mechanical strength when extracted from the mold. The technology of reaction injection molding is better able to produce small geometries. However, this is associated with difficulty in manufacturing small geometries without flash, which frequently dimensionally approaches the size of the contact separators or ribs. Additionally, since it is desirable to have a controlled characteristic impedance of the signal from the module to the printed circuit board without discontinuity, the use of plastics imposes a limitation, since the close proximity of the contacts results in cross talk therebetween. This cross talk problem may be alleviated by connecting alternate contacts to ground to provide a reference ground plane. However, this severely impacts the interconnection density which can be achieved. It is therefore a more specific object of this invention to provide a connector assembly between the IC chip module and a printed circuit board which has controlled impedance, wave guide properties, low electrical resistance, short electrical length, high density and reliability. Occasionally, an IC module becomes defective after installation on a printed circuit board. In such case, it is desirable to be able to replace such a module without requiring the replacement of the entire printed circuit board. Accordingly, it is a further object of this invention to provide a connector assembly which satisfies all of the foregoing requirements and, in addition, is separable to allow replacement of the module.